Underwater acoustic generator with variable resonant frequency



July 29, 1969 A. E. WALLEN 3,458,855

UNDERWATER ACOUSTIC GENERATOR WITH VARIABLE RESONANT FREQUENCY Filed April 23, 1968 5 Sheets-Sheet 1 Ma/or f m8 9 ALBERT E. WALLEN INVENTOR.

yam in M- A0 4 or/2e July 29, 1969 WALLEN 3,458,855

UNDERWATER ACOUSTIC GENERATOR WITH VARIABLE RESONANT FREQUENCY I Filed April 23, 1968 5 Sheets-Sheet 2 N A'BCDEF QH low Hague/76g 'q/1 56g aencg Hague/ray A LBERT E. Wfll-L EN 1 vcw ran 11/140411 rfkzu d /d M July 29, 1969 A WALLEN Q 3,458,855

UNDERWATER ACOUSTIC GENERATOR WITH VARIABLE RESONANT FREQUENCY Filed April 23. 1968 5 Sheets-Sheet s VFO ATT A July 29, 1969 A. E. WALLEN 3,458,855

' UNDERWATER ACOUSTIC GENERATOR WITH v VARIABLE RESONANT FREQUENCY Filed April 25, 1968 5 Sheets-Sheet 4 I t/fifl 2 I76 ALBERT E. WALLEN INVENT'OR AGENT July 29, 1969 Filed April 23, 1968 A. E. WALLEN TUBES FILLED X C-iM Q'UOZQ -IHIQHMOOCUI' O 6 a 9 \o u l2 \3 I4- \5 I6 n l8 a9 a0 2: 22 2a 24 2s 26 27 28 29 30 5e 59 so CALI BEAT ION BLANK 5 Sheets-Sheet 5 ALBERT E. WAL-l-E N INYflNTOR Assur- United States Patent 3,458,855 UNDERWATER ACUUSTIC GENERATOR WITH VARIABLE RESONANT FREQUENCY Albert E. Wallen, Winston Salem, N.C., assignor, by

mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed Apr. 23, 1968, Ser. No. 723,477 Int. Cl. 1104b 13/02 US. Cl. 340-5 Claims ABSTRACT OF THE DISCLOSURE The output frequency of an underwater acoustic generator employing bender-bar transducers is altered from the surface by causing compliant tubes, used in providing loading of the bender-bars, to be selectively filled with, or exhausted of, an incompressible fluid.

This invention relates to an improved underwater acoustic generator with a variable frequency output. More particularly, the invention relates to an underwater acoustic generator the output of which may be varied over several octaves by a remote control of the resonant frequency without removing the generator from the water.

It is known, in the prior acoustic generator art, to employ a grouping of compliant tubes to provide a loading of piezoelectric radiating elements. It is also known, in the prior art, to alter the resonant frequency of such units by altering the number of such tubes. Heretofore, this alteration required recovery of the underwater housing, disassembly of the housing, altering the number of tubes, reassembly, outgassing the insulating oil, repositioning the acoustic generator. This technique while technically satisfactory, requires the expenditure of considerable time ar'id effort.

The present invention overcomes the aforesaid laborious steps by selectively filling certain ones of the enclosed compliant tubes with the insulating oil. This effectively removes the tube from the generator without physically removing the tube. The selective filling and exhausting of certain tubes may be accomplished by employing commercially available control and fluid handling apparatus.

'It is, accordingly, an object of this invention to provide a method and apparatus for altering the frequency of an underwater acoustic generator.

More particularly, this invention has, as an object thereof, provision for altering the number of compliant tubes effective in loading a piezoelectric acoustic generator without disassembly of said acoustic generator.

A further object of the invention is the provision of a control system to alter the resonant frequency of a submerged bender-bar-type acoustic generator without recovery of the acoustic generator from its submerged location.

Another object of the present invention is the provision of a control system to permit the filling or exhausting of selected compliant tubes in a submerged acoustic generator from a remote surface control position.

Other objects and many of the attendant advantages will be readily appreciated as the subject invention becomes better understood by reference to the following detailed description, when considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a diagrammatic showing of the major components of the system of the invention showing a towed form of the acoustic generator;

FIG. 2 is a showing of an array of acoustic generators, each controlled in accordance with the invention, designed for fixed location;

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FIG. 3 is a simplified illustration, in quarter section, of a preferred form of the acoustic transducer of the systern;

FIG. 4 is an elevation of the mounting of a single bender-bar element;

FIG. 5 is a graphic representation of the output of the bender-bar transducer with different number of compliant tubes filled;

FIG. 6 is a schematic representation of a portion of the control system according to the invention;

FIG. 7 is a cross section of a control cable useful in connecting the control elements shown in FIG. 6 to a towed acoustic generator;

FIG. -8 is a schematic representation of a portion of the control system located at the acoustic generator including a simplified showing of the interconnection of several of the compliant tubes; and

FIG. 9 is a table of the order of filling of the compliant tubes shown in FIG. 8.

Referring to FIG. 1, there is shown an underwater acoustic generator housing 101 adapted to be towed beneath the surface of a body of water by means of an electrical conductor carrying tow cable 102. The housing encloses a forward bender-bar transducer 103 and an aft bender-bar transducer 104, shown by broken line outline. Identical transducers 103 and 104 are positioned behind acoustic windows, not shown, and their combined output is monitored by hydrophone 105 and suitable monitor readout circuitry 106. Hydrophone 105 is illustrated as being suspended below the generator housing 101. Such an arrangement is satisfactory, and even preferred, for accurate calibration, however, a mounting placing hydrophone 105 within the aft portion of the generator housing 101, as indicated at 109, is preferred for operational units. Similarly, the showing of the attachment is illustrative only and a more forwardly placed attachment is preferred for operational units as will be understood by proficient practitioners of hydrodynamic design. The transducers 103 and 104 are driven by an audio driver, shown diagrammatically at 107. Transducers 103 and 104 have a mechanism for filling and exhausting compliant tubes associated therewith. The filling and exhausting mechanism is controlled by suitable control means illustrated diagrammatically at 108.

The invention is not limited to towed or otherwise mobile acoustic generators, but may employ with equal facility stationary submarine arrays, such as that illustrated, for example, in FIG. 2. This arrangement employs four bender-bar acoustic generator assemblies 111 on a common axial support 112. Support 112 is incorporated in a framework 113 which also provides support for two reflectors 114 at approximately one-quarter wavelength distance from the transducer assemblies 111. Each of the generator assemblies 111 is equipped with apparatus, herein described, to vary its resonant frequency in response to electrical control signals carried by suitable cables 115. Cables 115 connect each generator assembly 111 with a remote, generally surface located, control point.

The particular housing and mounting structure depicted in FIGS. 1 and 2 are illustrative of the general types of mounting contemplated by the invention and are not considered limiting. Many of the known towed vehicle housings used in anti-submarine warfare and minesweeping arts, not resembling housing 101 of FIG. 1, are suitable for housing the apparatus of the invention. Similarly, the invention is useful in fixed installations other than that shown in FIG. 2, which resembles, in certain details, a Honeywell model S-1232A1 line array. However, for purposes of explanation, the general embodiment of FIG. 1 will be described.

FIG. 3 is a quarter section view of bender-bar transducer 103 of the invention, with some part removed for purposes of illustration. Within a protective shell 121 and an acoustic window 122, also serving as an outer casing, which are supported by pairs of annular rings 123 and 124, respectively, are positioned two mountings 125. A series of bender-bar supports 126, five shown, are positioned about the periphery of mountings 125. The sup ports 126 are arranged in pairs, one member of each pair on opposite mountings 125, so as to support a plurality of piezoelectric bender-bars 127.

As shown in FIG. 4, each bender-bar 127 is mounted so as to flex, as shown by broken lines 128, upon electrical excitation. The complete structure of the assembly of bender-bars 127, supports 126, and mountings 125 may be visualized as a wooden-barrel-like structure in which the staves, bender-bars 127, are caused to vibrate so as to vary the volume of the structure. When alternating current is applied to the bender-bars, they will all bend inward toward the compliant tubes on one-half of the sine wave, thus compressing the entrapped oil surrounding the compliant tubes. Spacing between the bars is about of an inch and will only allow a very small amount of oil to flow from the compliant tube chamber to the area between the bender-bars 127 and the acoustic window 122, this flow is not fast enough to provide pressure relief and the pressure will continue to build up until relieved by the compression of the compliant tubes. On the other half of the sine wave, all the bars bend outward releasing the internal pressure and creating a pressure between the bender-bars 127 and the acoustic window 122, thus establishing a pressure against the water surrounding the acoustic window 122. On the next half of the cycle, the first part is repeated again thus generating an alternating pressure wave in the Water. Filling the tubes with oil reduces the amount of internal pressure relief, thus restricting the oscillatory amplitude of the bender-bars which causes them to oscillate at a higher frequency. Although the structure illustrated is shown as providing for ten benderbar elements, the exact number of bender-bar elements may be altered to meet various design parameters. The bender-bars 127 of the preferred form of the invention are made of lead zirconate, but, as in the case of the number of bars, the skilled acoustic generator artisan may make appropriate substitutions without departing from the teachings of the invention.

Again referring to FIG. 3, in the center region of the bender-bar transducer 103 are located compliant tubes 130. An inner separator 129 surrounds the compliant tubes 130. Perforations 131 in the inner separator 129 permit the entire cavity within casing 122, including the interstices between individual ones of compliant tubes 130, to be filled with a suitable incompressible insulating oil. End plates 132, mounted on rings 124, make the cavity oil tight. Compliant tubes 130 have closed ends except for interconnecting tubing which join selected groups of compliant tubes 130 into a series of chambers. Each of the groups of compliant tubes 130 are connected to a fluid supply tube, shown at 133, and a vent tube, such as shown at 134. It should be understood that each of the groups of tubes have associated therewith a fluid supply tube 133 and a vent tube 134. However, for purposes of illustration only a single tube of each type is illustrated and described. Feed through terminals 135 permit electrical connection to the piezoelectric bender-bar elements 127, within the casing 122, by suitable internal wiring, not shown, and bender-bar supports 126. A series of perforations 136 permit contact with the outer, acoustically-transparent casing 122 by fluid within which the transducer is submerged, while permitting protective shell 121 to prevent mechanical contact between outer casing 122 and large objects. Shell 121 may be the outer surface of a towed acoustic generator housing 101. However, if desired, other mounting structure, such as apertured flange 137, may be employed.

It has been observed in prior art bender-bar transducers that the removal of a number of compliant tubes increases the resonant frequency of the bender-bar element. However, the relationship of the number of tubes removed to the change in frequency is not linear. The smaller the number of tubes remaining the greater the frequencydeviation-per-tu'be becomes. The proximity of the removal compliant tube to the bender-bar element is also controlling on the frequency change produced by the removal. The nearer the compliant tube to the bender-bar the more pronounced is the change. Of course, general expressions have been developed for the relationships noted above but the variations between units has made the arrival at a precise frequency a rather time-consuming, empirical task.

By means of a number of tube pairs 133 and 134 and apparatus herein described, the selected ones of compliant t-ubes may be filled with the same insulating oil as surrounding the tubes, and thereby effectively removed without physical removal. The acoustic output energy of the device with sequential filling of selected groups of compliant tubes 130 is represented by FIG. 5.

In FIG. 5, the left-most curve, labeled normal, represents the output of the device with all tubes gas filled. As shown, the output of the bender-bar transducer exhibits a rather broad peak. Because the bender bars 127 will tend to oscilalte at the lowest possible frequency, the filling of selected groups of compliant tubes 130, which may be interconnected to form fluid circuits, with fluid shifts the curve up the frequency scale. It should be understood that the device operates on only a single one of the nearly identical curves shown in FIG. 5 at a time. As the resonant frequency increases a lesser amount of drive power is required to make the individual bender-bar elements 127 achieve the maximum deflection permitted by the reduced compliance afforded by the fluid filled tubes. This reduced deflection amplitude is offset, somewhat, by an increased coupling efliciency at the higher resonant frequency. As a result, a linear power output over the operational range is obtainable with only minor adjustments of the driving power and appropriate selection of compliant tubes to be filled.

The precise number of circuits is a matter of design dictated by the requirements of the particular installation. The simplest embodiment would be a two frequency system in which a certain number of the compliant tubes would be filled to shift the resonant frequency. With only a moderate increase in complexity over the two frequency system, a more comprehensive system may be provided.

The control and instrumentation of the audio generator is accomplished by the apparatus and circuit arrangement shown in FIG. 6. The circuit and apparatus diagram is divided into three sections, an audio driver section 107, a monitor section 106 and a motor and valve control section 108. As shown, the audio driver section 107 includes a variable frequency oscillator 138. The variable frequency oscillator 138 is connected to an attenuator 139. The output of attenuator 139 is connected to amplifier 141, a 5 kva. amplifier. The output of amplifier 141 is connected via a wattmeter 142 to meter dividers 143 and 144 and those to a terminal strip 148. This connection arrangement permits monitoring the audio driver circuit 107 while the two individual transducers are driven an arrangement which has proven optimum for acoustic generator operation. Meter dividers 143 and 144 are frequency compensated voltage dividers and current transformers. They reduce the voltage and current 1000/1 for safe monitoring on a VTVM. The electrical signal is fed from meter dividers 143 and 144 to suitable measuring devices, dual trace oscilloscope 145 and vacuum tube voltmeters 146 and 147, which provide an indication of the electrical drive signal fed to each of the individual bender-bar assemblies 103 and 104. The output of audio driver circuit 107 is, as noted above, fed to a suitable terminal strip 148 and is carried therefrom by conductors 151, 152, and 153 as two in-phase signals having a common conductor 152.

The acoustic energy radiated from acoustic generator housing 101 impinges hydrophone 105, which as previously noted may be mounted within acoustic generator housing 101, to produce an electrical output which is transmitted via shielded conductors 154 and 155 to terminal strip 148 and monitor circuit 106. The electrical signal carried by conductors 154 and 155 is fed to a repeater coil 156 which, in turn, feeds a suitable indicator such as a vacuum tube voltmeter 157. A conductor 158 from acoustic generator 101 is connected to terminal strip 148 and then to a suitable common ground 159. Although shown as a portion of the monitor circuit 106, the conductor 158 and common ground 159 serve as a reference and DC return for the audio driver circuit 107, monitor circuit 106, and motor and valve control circuit 108.

Referring to the motor and valve control circuit 108 shown in the lower portion of FIG. 6, DC power is brought into terminal strip 148 by conductors 161 and 162. A three pole switch 163, which has a neutral center position, has two of its poles connected to conductors 161 and 162 to have a selective, polarity-reversed output. The output of the polarity-reversing position of switch 163 is routed to terminal strip 148 and thence, via conductors 164 and 165, to a reversible motor in acoustic generator 101. The current drain of said motor is monitored by a reversible DC ampmeter 166.

The faces of meter 166 has two calibration markings in each current direction, the manner in which the marking serve their intended function will be explained in discussion of the motor and valve apparatus. Mark 167, near the full-scale point, is used in calibration of the generator and may be suitably labeled, if desired. An intermediate marking 168 on one polarity scale is used to indicate that the filling of one group of compliant tubes 130 is completed and may be suitably labeled if desired. Marking 169, nearest the zero-current position on the opposite polarity scale, is used to indicate that one group of compliant tubes 130 are emptied of fluid and may be suitably labeled, if desired.

The third pole of switch 163 is used to operate suitable valve mechanisms, to be later described, by connecting conductor 171, via terminal strip 148, to the positive DC power conductor 161, when switch 163 is in either of extreme positions.

A second switch 172 is operated by a button 173 which is mounted on the panel 174, which may comprise part of the housing of the motor and valve control 108. Button 173 also rotates indicia bearing drum 175 past a viewing aperture 177 by suitable linkage not shown. Upon each closure, switch 172 operates a distribution valve, to be later described, by connecting conductor 176, via terminal strip 148, to the positive DC power conductor 161. Like conductors 154 and 155, conductors 164 and 16 5, and conductors 171 and 176, are contained in suitable shielding to prevent the various audio frequency and DC switching circuits from interfering with one another. As previously noted, the electrical conductors joining the control circuitry and audio generator 101 are carried within tow cable 102.

Referring to FIG. 7, tow cable 102 is, preferably, covered with a double layer of armor cable. The outer layer 178 of armor is comprised of twenty-four strands of .070 diameter steel wire in a left hand lay. The inner layer 179 of armor comprises twenty-four strands of .055 wire in a right hand lay. The electrical conductors are contained in an insulation material 181, which may be made of a material known under the trade name Permagum, surrounded by an outer, waterproof layer 182. Layer 182 is made of polyethylene and has a thickness of .045 inch. The individual conductors 151, 152, and 153 are, preferably, No. 16 AWG 7 tinned wire with .058 inch insulating layers 183, 184, and 185, of polyethylene suitably color coded. Conductor 158 may be, preferably, a number of 20 AWG wire and is centered between conductors 151, 152, and 153. Conductor 158 serves as the instrumentation round return. The shielded conductor pairs 154, 164, and 171, 176 are spaced between layer 182 and the conductors 151, 152., and 153 in the space between the individual conductors 151, 152, and 153. Conductors 158, 164, 165, 171, and 176, used in the control of the filling and exhausting of compliant tubes 130, terminate within audio generator housing 101, in a suitable connection means.

Referring to FIG. 8, contained within mounting 125 are shown a plurality of compliant tubes 1-71. It should be understood that more tubes are generally used but are grouped into clusters with common connections to the filling and exhausting mechanism, however, for purposes of illustration and explanation it is suflicient to consider the transducer as including seventy-one compliant tubes as illustrated in FIG. 8. The compliant tubes are connected in fluid circuits by suitable internal tubes 191. In the example of FIG. 8, only a single circuit comprising individual compliant tubes 1, 2, 3, 4, and 5 is illustrated, but the remaining tubes are grouped in similar circuits, as will be explained. In the system of FIG. 8 the seventy-one tubes are connected in twenty-one fluid circuits; nine with five compliant tubes each; seven with three; and five with one.

The fluid circuit is connected by tube 133 to a distribution valve 192 and by tube 134 to a distribution valve 193. Valves 192 and 193 are physically identical and, for purposes of explanation, only the former will be described in detail.

Valve 192 has a cylindrical center portion 194 which is fitted in, and rotated with respect to, an outer portion 195. The outer portion 195 has twenty-one fluid passing connections 196 (three shown) and a single blank space 197. Center portion 194 fits outer portion 195 with a fluid tight fit and, thereby, seals all of fluid passing connections except one which communicates with a fluid passage 198 in center portion 194. Center portion 194 is mechanically joined to the center portion of valve 193 in such a manner that the two valves align with corresponding fluid passing connections. The two valve assemblies 192 and 193 may be made integrally, if so desired.

Center portion 194 and the center portion of valves 193 are mechanically joined to a twenty-two tooth ratchet 201. This ratchet 201 cooperates with a pawl 202 mounted on plunger 203 of solenoid 204. One electrical lead of solenoid 204 is connected to ground point 205 and the other electrical lead 206 is joined to conductor 176 at terminal strip 207. Terminal strip 207 is shown for purposes of illustration and, like terminal strip 148, may be any suitable multi-wire electrical connection, if desired.

From center portion of valve 193 a fluid conduit 208 provides a passage for the oil to a fluid reservoir 209 via relief valve 210 and isolation valve 211. Relief valve 210 is bypassed for gas flow by a check valve 231. Isolation valve 211 is paired with a similar isolation valve 212 which is fed by fluid conduit 213 from the other benderbar transducer 104. Both isolation valves 211 and 212 are actuated by common solenoid 214.

From fluid reservoir 209 the oil is conducted by conduit 215 to a reversible pump 216. The output of pump 216 is conducted via conduit 217 through isolation valve 218 and conduit 221 to distribution valve 192. Isolation valve 218 is paired with isolation valve 219 to provide a connection from conduit 217 to bender-bar transducer 104 via conduit 220. Isolation valves 218 and 219 are actuated by solenoid 222. Solenoids 214 and 222 have one lead each connected to electrical conductor 223 which is joined to electrical conductor 171 at terminal strip 207. The other leads from solenoids 214 and 222 are grounded at points 224 and 225 respectively. Electrical conductor 158 is connected by terminal strip 207 to a ground point 226.

Pump 216 is bypassed by a suitable two-way relief valve 227, the relief pressure thereof is greater than the relief pressure of relief valve 210. A reversible DC motor 228, connected by suitable electrical conductors 229 and 7 230 to electrical conductors 164 and 165 at terminal strip 207, drives pump 216 in a direction determined by the position of switch 163 (FIG. 6) to either fill or exhaust the particular compliant tube circuit selected by distribution valves 192 and 193.

As previously mentioned, the shift of resonant frequency is proportional to the percentage of the compliant tubes 130 filled and inversely to the distance the particular compliant tube is removed from the bender-bar elements. Assuming it is desired to increase the resonant frequency in the most nearly linearly related steps, the tubes are grouped into circuits such that a plurality of individual compliant tubes near the bender-bar elements are filled first. With each depression of button 173, selector valves 192 and 193 connect another group of compliant tubes near the bender-bar elements and symmetrically positioned with respect to the compliant tubes comprising the circuit previously filled. Each depression of button 173 also rotates drum 175 to expose sequentially related indicia in viewing aperture 177. Each indicium corresponds to a particular position of distribution valves 192 and 193 and therefore to a particular tube circuit. The tube circuits corresponding to the twenty-one positions of the device of FIGS. 6 and 8 are given in the chart of FIG. 9. After selecting the desired circuit of complaint tubes with button 173, switch 163 is moved in the appropriate direction to fill or exhaust the circuit as desired. With the use of a calibration chart of the general form shown in FIG. 5, the transducer may be tuned to the particular resonant frequency desired with the use of button 173 and switch 163. To reduce the resonant frequency, it is necessary to depress button 173 twentyone times for selection of the previously filled circuit before throwing switch 163. In practice the resonant frequency is not altered so frequently as to make this operation troublesome, but a reversible selector valve actuator could be incorporated by the use of normal skill at some increase of connecting cable complexity.

When switch 163 is operated to energize motor 228, solenoids 214 and 222 open the isolation values to permit a complete fluid circuit to fill or exhaust the tubes in both transducers 103 and 104. In the position of distribution valves 192 and 193 shown in FIG. 8, tubes 1, 2, 3 ,4 and 5 are filled in sequence and the oil then passes through relief valve 210 into reservoir 209. Relief valve 210 is held open by the action of pump 216. When relief valve 210 and the corresponding valve associated with benderbar transducer 164 are held open, the load on motor 223 stabilizes to a value determined by the relief pressure of the valves. This load is monitored on meter 166 (FIG. 6) and calibration marking 168 corresponds to this load.

When the fluid circuit selected by distribution valves 192 and 193 is desired to be exhausted, switch 163 is placed in the appropriate position to cause the pump 216 to move the oil from the compliant tubes to fluid reservoir 209 via conduits 133, 221, and 215. The gas in reservoir 209 is forced out, via conduit 208, by the incoming fluid and bypasses relief valve 210 through check valve 231. When the oil is exhausted from the circuit, pump 216 cavitates and offers only a light load to motor 228. The calibration marking 169 on meter 166 indicates that this condition has been obtained.

When the blank space 197 is selected no oil can flow through conduits 217 and 221 or valve 218. The pressure build-up, when the pump is activated with valves 192 and 193 in this position, opens valve 217 to bypass pump 216. The load on motor 228 is at a maximum for this condition and is indicated b marks 167 on meter 166. Markings 167 are used to calibrate the distribution valves 192 and 193 and indicia wheel 175 in the following manner: First the button 173 then switch 163 (normally in the full direction) are pressed serially until a reading on meter 166 indicates that the calibration position has been reached, then the connection between conductor 176 and switch 172 is interrupted. Indicia bearing drum 175 is then rotated until the indicium corresponding to the calibration position appears in viewing aperture 177. The connection between conductor 176 and switch 172 is then reestablished.

The operation of the audio driving circuitry is straightforward. The desired frequency is set on the variable frequency oscillator 138. The attenuator 139 is adjusted to provide the necessary driving power as indicated on wattmeter 142. Meters 146 and 147 and dual trace oscilloscope provide the desired instrumentation of the power to each of the bender-bar transducers 103 and 104. The above operational adjustments, together with the changing of the resonant frequency of the transducer, are assisted by the monitor circuit 106.

The foregoing description is applicable to the towed acoustic generator of FIG. 1. If the fixed array of FIG. 2 is to be employed, it is obvious that the apparatus will be modified. Each of transducer assemblies in acoustic generators 111 would require its own pumping and valve apparatus such as shown in FIG. 6, with the exception that valves 212 and 219 would not be required. The control circuits for the pumping and valve apparatus could be shared. The audio drive circuit of FIG. 6, likewise, could be modified to provide the drive energy for four, rather than two, bender-bar transducers. Such modifications, although not specifically illustrated and described herein, are considered within the scope of competence of a proficient artisan employed in the electro-acoustic industry having the benefit of the teaching herein set forth.

Obviously, other embodiments and modifications of the subject invention will readily come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing description and the drawings. It is, therefore, to be understood that this invention is not to be limited thereto and that said modifications and embodiments are intended to be included within the scope of the invention.

What is claimed is:

1. An underwater acoustic generator comprising:

a casing having acoustically transparent, fluid impervious walls;

an electrically insulating fluid filling said casing;

at least one electroacoustic transducer located within said casing and immersed within said electrically insulating fluid;

a plurality of compliant tubes located within said casing in such a fashion as to be immersed within said electrically insulating fluid and operatively coupled thereby to said electroacoustic transducer; and

fluid supply means located outside said casing and operatively joined to said compliant tubes to selectively fill or exhaust predetermined ones thereof.

2. An underwater acoustic generator according to claim 1 further comprising:

a plurality of fluid conducting tubes connected between predetermined individual ones of said compliant tubes for forming a plurality of fluid circuits.

3. An underwater acoustic generator according to claim 2 in which said fluid supply means includes a pump driven by an electric motor.

4. An underwater acoustic generator according to claim 3 in which said fluid supply means includes distribution valve means connecting each of said fluid circuits successively to said pump.

5. An underwater acoustic generator according to claim 4 in which said distribution valve means has one position isolating said pump from any of said fluid circuits.

6. An underwater acoustic generator according to claim 5 in which said fluid supply means inlcudes additional valve means to isolate said compliant tubes during periods when said pump is inoperative.

7. An underwater acoustic generator according to claim 6 further comprising:

a source of electrical energy to drive said electroacoustic transducer element;

an electrical control circuit for controlling said fluid supply means;

a housing means enclosing said casing and said fluid supply means; and

waterproof electrical conductor means providing operative electrical connection from said source of electrical energy and said electrical control circuit to said electroacoustic transducer means and said fluid supply means, thereby permitting said source of electrical energy and said electrical control circuit to be located above the surface of a body of water when said acoustic generator housing means is submerged within said body of water.

8. An underwater acoustic generator according to claim 7 in which said control circuit includes means to measure the electrical current used by said electric motor.

9. An underwater acoustic generator according to claim 8 in which said control circuit further includes means to control the operation of said distribution valve means to effect the sucecssive connection of said fluid circuits to said pump.

10. An underwater acoustic generator according to claim 9 in which said means to control the operation of said distribution valve means additionally indicates to which of said fluid circuits said pump is connected.

References Cited UNITED STATES PATENTS RICHARD A. FARLEY, Primary Examiner US. Cl. X.R. 3408, 10 

